T\lymphocyte-derived cell lines, including cytolytic T\lymphocyte (CTL) clones possessing stable lytic activity, have been established from mouse lymphocytes. Variant CTL lines having full, partial, aberrent and null lytic activity have been established. Two approaches are currently being used for obtaining variant CTL lines. The first approach uses a panel of monoclonal antibodies and complement to select antigenic-loss variants deficient in lymphocyte surface antigens such as LFA-1, Lyt 2/3 H-2, Thy 1, and T-200. The second approach selects functional variants by screening for altered lytic activity using 51Cr release assays, and may result in variants deficient in as yet unidentified molecules, such as the receptor and lysin. For variants showing aberrant activity we are determining the affected step: motility, recognition, postrecognition adhesion, and/or programming for lysis. With quantitative flow cytofluorometry and monoclonal antibodies we are analyzing antigenic phenotypes of clones. Subsequently, the molecular nature of altered molecules will be studied by comparing membrane preparations from variant, parental, and non-CTL lines using a number of techniques such as immunoprecipitation, SDS-PAGE, isolectric focusing and two-dimensional gels. By correlating the presence or absence of discrete CTL activities such as motility, recognition, adhesion, and programming for lysis with mutations affecting cell surface molecules, we expect to identify and characterize molecules involved in CTL-mediated killing.